This invention relates to an apparatus for measuring the elevation of a three-dimensional foreground subject and more particularly to an apparatus for measuring the elevation of a three-dimensional foreground subject by reference to paired images of said foreground subject photographed from two points, of known height (said photographed images are generally referred to as "stereoscopic photographs").
The known means for preparing a contour map utilizes a plurality of aeronautic stereoscopic photographs. These stereoscopic photographs are derived from the photographing of substantially the same area from two points of known height spaced from each other at a prescribed distance. Namely, the prior art process comprises the steps of optically treating the stereoscopic photographs of a three-dimensional foreground subject determining the displacements of corresponding points included in a pair of said stereoscopic photographs and calculating the elevation of the foreground subject by said displacements and the known height of said different points of photographing and a distance therebetween. However, the process of determining the elevation of a foreground subject simply by optically treating the corresponding points on the images photographed at two points, namely determining said elevation from density data merely consisting of image elements, that is, data on the light and shade of the photographed images presents difficulties in accurately measuring said elevation. General practice, therefore, is to determine the interrelationship of the minute areas including the neighborhood of the above-mentioned elements indicated in the photographs, regard those areas whose interrelationship gives a maximum value as mutually corresponding ones and calculate the displacement of said minute areas from each other as that of the corresponding points.
However, the above-mentioned customary practice fails to carry out the accurate measurement of the elevation of a three-dimensional foreground subject, because, as any selected points in the minute areas in the stereoscopic photographs of said foreground subject denote the different elevation of said foreground subject, distortion takes place in said stereoscopic photographs.